A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When multiple patterns are transferred subsequently to a substrate, it may be desired to align subsequent patterns relative to each other. To align a subsequent pattern to a previously transferred pattern it is desirable to know the location of the previously transferred pattern. In order to determine the location of a pattern on a wafer, alignment marks are transferred to predefined positions on the substrate. By measuring the position of the alignment marks, information can be obtained that can be used to transfer a subsequent pattern relative to the previously transferred pattern to the substrate.
The position information of a previously transferred pattern, which is used to accurately transfer a subsequent pattern relative to the previously transferred pattern, usually does not correspond one to one to the position information obtained from measuring the position of the alignment marks, as not all areas of a pattern can be used to place alignment marks. As a consequence, alignment marks are usually placed at edges of a pattern or in so-called scribe-lanes, while it is important that the center regions of the pattern used to manufacture devices are aligned with respect to each other.
To solve this, a model can be fitted to the measured positions of the alignment marks. This model can be used to determine the position information of a previously transferred pattern that can be used to accurately transfer a subsequent pattern relative to the previously transferred pattern.
In an attempt to more accurately determine the position information of a previously transferred pattern, advanced alignment models, like higher-order alignment models, (extended) zone alignment and grid alignment, are used. However, the success of these models is dependent on the distribution and number of alignment marks.
However, using a large number of alignment marks has a negative impact on the throughput of the lithographic apparatus. Furthermore, the known advanced alignment models may be very sensitive to outliers.